KR20100076073A

KR20100076073A - Steel sheets and process for manufacturing the same

Info

Publication number: KR20100076073A
Application number: KR1020107013257A
Authority: KR
Inventors: 노부스케 가리야; 가즈히로 세토
Original assignee: 제이에프이 스틸 가부시키가이샤
Priority date: 2007-12-19
Filing date: 2008-11-20
Publication date: 2010-07-05
Also published as: EP2246450A4; WO2009078261A1; KR20130035276A; CN101903547A; CN101903547B; EP2246450B1; EP2246450A1

Abstract

Provided are steel sheets which are soft and have excellent workability and excellent hardenability and a process for manufacturing the same. A steel sheet characterized by having a composition which contains by mass C: 0.3 to 0.7%, Si: 0.1% or below, Mn: 0.20% or below, P: 0.01% or below, S: 0.01% or below, Al: 0.05% or below, and N: 0.0050% or below with the balance being Fe and unavoidable impurities and a structure which comprises ferrite, graphite and cementite and in which the total volume fraction of ferrite, graphite and cementite accounts for at least 95% of the whole structure and the ratio of the volume of graphite to the total volume of graphite and cementite (degree of graphitization) is 5% or above, wherein the mean particle diameter of graphite and cementite is 5μm or below; and a steel sheet characterized by having the above composition and the above structure, wherein the sum of the volume fractions of graphite and cementite present in ferrite grains accounts for 15% or below of the total of graphite and cementite.

Description

Steel plate and its manufacturing method {STEEL SHEETS AND PROCESS FOR MANUFACTURING THE SAME}

TECHNICAL FIELD The present invention relates to a steel sheet suitable for use in automobile parts and the like, in particular, a steel sheet excellent in workability and hardenability, and a method of manufacturing the same.

Steel sheets used for tools or automobile parts (gears, missions) and the like are often used after being processed into a desired shape, followed by heat treatment such as quenching and tempering. Since such steel sheets are processed into various complicated shapes, excellent workability is required. In recent years, there has been a strong demand for reduction of manufacturing costs for such components, and it is possible to increase the machining technology for the purpose of eliminating the machining process or changing the machining method, for example, thickening of automotive drive system components using high carbon steel sheets. In this way, double-acting processing technology and the like, which have realized a significant shortening of the process, have been developed and partially used. As a result, the demand for workability is increasing in steel sheets used in automobile parts and the like, and are required to be softer and higher in ductility. For example, when processing by cold forging, lower yield stress is calculated | required. Moreover, when carrying out hole expansion process (burring) after a punching process, the outstanding stretch flange property is calculated | required.

In order to meet these demands, the technique of graphitizing C in steel and improving workability is examined. For example, Patent Document 1 has a mass% of C: 0.40 to 0.80%, Si: 0.20 to 2.00%, Mn: 0.20 to 1.50%, Al: 0.001 to 0.150%, P: 0.018% or less, and S: 0.010. % Or less, N: 0.0050% or less, consisting of residual Fe and inevitable impurities, and having a structure mainly composed of a ferrite phase and a graphite phase, and having TS

A steel sheet suitable for a tiller claw part having good workability, toughness and hardenability having a soft material of 60 kgf / mm 2 and a manufacturing method thereof is disclosed. Moreover, in patent document 2, in mass%, C: 0.10 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 0.50%, Nb: 0.005 to 0.1%, Al: 0.01 to 1.00%, N: 0.002 to 0.010%, B: 3-50 ppm, Ca: 0.001-0.01%, and Ni: 0-2.00%, the remainder consisting of Fe and inevitable impurities, P: 0.012% or less in impurities, S: 0.008% less than the hot-rolled steel sheet, and to the mixture was kept Ac ₁ ~Ac 0.1~10 hr at a temperature ranging from _three points, the cooling rate of 20~100 ℃ / hr cooling to ambient temperature, followed by box annealing at a temperature of 650~750 ℃ The manufacturing method of the medium-carbon steel plate excellent in the workability characterized by graphitizing 50 area% or more of cementite in steel is disclosed. Moreover, in patent document 3, by mass%, C: 0.20 to 1.00%, Si: 0.20% or more, 1.20% or less, Mn: 0.05 to 0.50%, N: 0.005-0.015%, B: 0.2 x N%-0.8 x N%, and Al: less than 0.05%, and contain an amount satisfying 1.0 × (NB)% to 5.0 × (NB)%, and are composed of residual Fe and unavoidable impurities, and as impurities, P: 0.020% or less , S: 0.010% or less, a high-carbon steel sheet having good workability, having a chemical composition of ferrite, graphite, and cementite, and a manufacturing method thereof are disclosed.

Japanese Patent Laid-Open No. Hei 1-025946 Japanese Laid-Open Patent Publication No. 7-258743 Japanese Laid-Open Patent Publication No. 4-202744

Conventionally, in order to improve the workability by graphitizing C in steel, as described in patent documents 1, 3, etc., addition of a large amount of Si became essential. However, by adding Si, the ferrite itself becomes hard and it is difficult to obtain good workability. In addition, as in Patent Document 2, a component system containing B and Nb is added, and by performing annealing twice under predetermined conditions, a technique for achieving graphite and high ductility without developing a large amount of Si addition is also developed. However, performing two annealing leads to an increase in cost. Patent Literature 2 is a technique for graphitizing at least 50% of cementite in steel, and as the component composition of the steel disclosed in Examples of Patent Literature 2, the amount of Si is large, and the amount exceeds 0.20%. . In addition, although the steel sheets described in Patent Documents 1 to 3 are soft and excellent in bending workability and elongation characteristics in a tensile test, graphite and cementite do not sufficiently dissolve depending on heating conditions at the time of quenching of the steel sheet. In some cases, poor quenching may occur. Moreover, although the steel plates of patent documents 1-3 are soft, there existed a problem that elongation flange property which is an index about the hole expansion workability after a punching process was not necessarily excellent.

An object of the present invention is to provide a steel sheet which is soft, has excellent workability, has excellent hardenability, a steel sheet having excellent workability having excellent elongation flangeability, and a method for producing the same.

MEANS TO SOLVE THE PROBLEM As a result of repeatedly examining the subject of the said prior art, even when the content of Si is made very low, specifically 0.1% or less in high carbon steel, by necessarily controlling distribution of graphite and cementite, It was found that even without increasing the graphitization rate, good workability can be obtained and good hardenability and elongation flangeability can be ensured. That is, as a result of earnestly researching the effect of the structure on the strength, hardenability, and elongation flange property of the steel plate containing C: 0.3-0.7 mass%, the following things were found.

(1) For soft nitriding, a structure containing ferrite, graphite, and cementite is used. The total volume fraction of ferrite, graphite, and cementite in the entire structure is 95% or more, and the volume fraction of graphite in the graphite and cementite as a whole is 5%. It is effective to make more than%.

(2) In order to improve hardenability, the average particle diameter of graphite and cementite must be 5 micrometers or less.

(3) Cooling conditions after hot rolling are very important for controlling the particle sizes of graphite and cementite.

(4) In order to improve the stretch flangeability, the total volume fraction of graphite and cementite present in the ferrite grains occupying the entire graphite and cementite must be 15% or less.

(5) Cooling conditions after hot rolling are very important for controlling the volume ratio of graphite and cementite present in the ferrite particles.

This invention is made | formed based on such knowledge,

In mass%, C: 0.3-0.7%, Si: 0.1% or less, Mn: 0.20% or less, P: 0.01% or less, S: 0.01% or less, Al: 0.05% or less, N: 0.0050% or less, and It has a composition consisting of balance Fe and unavoidable impurities, has a structure containing ferrite, graphite, and cementite, and the total volume fraction of ferrite, graphite, and cementite in the whole structure is 95% or more, and graphite in the whole graphite and cementite A steel sheet characterized by having a volume fraction (graphite ratio) of 5% or more and an average particle diameter of graphite and cementite of 5 µm or less.

It is preferable that the steel sheet of this invention contains at least 1 sort (s) further from mass: Ni: 3.0%, B: 0.005% or less, Cu: 0.1% or less by mass%.

The steel sheet of this invention hot-rolls the steel which has the said composition at the finishing temperature of 800-950 degreeC, and makes it a hot rolled sheet, and cooling the hot rolled sheet after the said hot rolling at 500 degrees C or less at the average cooling rate of 50 degrees C / s or more. After cooling to a stop temperature, it can wind up at the winding temperature of 450 degrees C or less, and can manufacture by the method of annealing at the annealing temperature of 720 degrees C or less after the said hot rolled sheet.

The present invention,

In mass%, C: 0.3-0.7%, Si: 0.1% or less, Mn: less than 0.15%, P: 0.01% or less, S: 0.01% or less, Al: 0.05% or less, N: 0.0050% or less, and It has a composition consisting of balance Fe and unavoidable impurities, has a structure containing ferrite, graphite, and cementite, and the total volume fraction of ferrite, graphite, and cementite in the whole structure is 95% or more, and graphite in the whole graphite and cementite The steel sheet characterized by the volume fraction (graphite rate) of 5% or more and the sum of the volume fractions of graphite and cementite present in the ferrite particles occupying the entire graphite and cementite is 15% or less.

The steel sheet of this invention hot-rolls the steel which has the said composition at the finishing temperature of 800-950 degreeC, and makes it a hot rolled sheet, and cooling the hot rolled sheet after the said hot rolling to 600 degrees C or less at the average cooling rate of 50 degrees C / s or more. After cooling to a stop temperature, it can wind up at the winding temperature of 550 degreeC or less, and can manufacture by the method of annealing at the annealing temperature of 720 degreeC or less after the said hot rolled sheet.

According to the present invention, a steel sheet which is soft and has excellent workability and has excellent hardenability can be produced. In particular, since the steel sheet of this invention only needs to control the component and the cooling conditions after hot rolling, it can manufacture easily and inexpensively. Moreover, since the steel plate of this invention is soft and excellent in workability, it is suitable for thickening and processing of automotive drive system components, and even if it is applied to components of a complicated shape, processing and welding of a plurality of components become unnecessary, and the productivity improvement of automobile components is Cost reduction can be aimed at. In addition, in the steel sheet of the present invention, hardening defects caused by undissolved graphite and cementite do not occur during heating at high frequency or the like.

According to the present invention, a steel sheet which is soft and has excellent workability having excellent elongation flangeability can be produced. In particular, since the steel sheet of this invention only needs to control the component and the cooling conditions after hot rolling, it can manufacture easily and inexpensively. Moreover, since the steel plate of this invention is soft and excellent in workability, such as elongation flange property, it is suitable for thickening and processing of automotive drive system components, and even if it is applied to components of a complicated shape, processing and welding of multiple components become unnecessary, The productivity of parts and cost reduction can be aimed at.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the relationship of the average particle diameter d and (DELTA) Hv of cementite and graphite.
FIG. 2 is a diagram showing the relationship between the volume fraction S and average λ of cementite and graphite present in ferrite particles.

Best Mode for Carrying Out the Invention

Below, the steel plate excellent in the workability which is this invention, and its manufacturing method are demonstrated in detail. In addition, "%" which shows the quantity of a component means "mass%" unless there is particular notice.

1) Composition

C: 0.3-0.7%

C is an element which forms graphite. If the amount of C is less than 0.3%, the hardness after quenching cannot be secured. If the amount of C is more than 0.7%, even if graphite is formed, the steel sheet is hardened and workability is lowered. For this reason, the amount of C is made into 0.3 to 0.7%.

Si: 0.1% or less

When the amount of Si exceeds 0.1%, ferrite hardens and workability falls. For this reason, Si amount is 0.1% or less, Preferably it is 0.05% or less.

Mn: 0.20% or less

If the amount of Mn exceeds 0.20%, graphite formation is inhibited, so Mn is 0.20% or less, preferably 0.10% or less.

P: 0.01% or less

Since P segregates in grain boundaries and reduces workability, and P has a function of stabilizing cementite and inhibiting graphite formation, it is preferable to reduce P as much as possible. For this reason, P amount is 0.01% or less, Preferably you may be 0.008% or less.

S: 0.01% or less

S forms sulfides such as MnS to lower workability, and since S has a function of stabilizing cementite and inhibiting graphite formation, it is preferable to reduce S as much as possible. For this reason, the amount of S is made into 0.01% or less, Preferably it is 0.007% or less.

Al: 0.05% or less

Al is an element which combines with solid solution N to form AlN, harms the adverse effect of solid solution N having a function of inhibiting graphite formation, and promotes graphite formation using AlN as a nucleus.

For this reason, it is preferable to make Al amount into 0.003% or more. When it exceeds 0.05%, since cleanliness of steel will fall and workability will deteriorate, Al amount shall be 0.05% or less, Preferably it is 0.04% or less.

N: 0.0050% or less

When the amount of N exceeds 0.0050%, the effect | action which stabilizes the cementite of solid solution N becomes remarkable, and graphite formation is inhibited. For this reason, N amount is made into 0.0050%, Preferably it is 0.0040% or less.

The balance is Fe and an unavoidable impurity. For the following reasons, at least one selected from Ni: 3.0% or less, B: 0.005% or less, and Cu: 0.1% or less is preferably contained.

Ni: 3.0% or less

Ni is an element that promotes graphite formation and is also an effective element for improving hardenability, and in order to obtain such an effect, Ni is preferably contained at 0.1% or more. When the amount of Ni exceeds 3.0%, the effect is saturated. For this reason, Ni amount is 3.0% or less, Preferably it is 0.1 to 3.0%, More preferably, you may be 0.3 to 1.0%.

B: 0.005% or less

B is a useful element that combines with N to form BN and acts as a nucleus for graphite formation, and is also an effective element for improving hardenability, and it is preferable that B is contained at 0.0005% or more in order to obtain such an effect. When the amount of B exceeds 0.005%, the effect is saturated. For this reason, B amount is 0.005% or less, Preferably it is 0.0005 to 0.005%, More preferably, you may be 0.0010 to 0.0040%.

Cu: 0.1% or less

Cu is an element which promotes graphite formation and is also an effective element for improving hardenability, and in order to obtain such an effect, Cu is preferably contained at 0.01% or more, more preferably 0.02% or more, and the amount of Cu exceeds 0.1%. If so, the effect is saturated. For this reason, Cu amount may be 0.1% or less, More preferably, you may be 0.07% or less.

2) organization

In order to soften the steel sheet and to improve the bending property and the elongation characteristics in the tensile test, a structure containing ferrite, graphite, and cementite is used, and the sum of the volume fractions of ferrite, graphite, and cementite in the entire structure It is necessary to make it 95% or more, and to make the graphite rate occupied in the graphite and cementite whole 5% or more. At this time, in the present invention, the same effect is obtained even when the graphite rate is 100%, that is, all the cementite is graphite, and therefore, it will be included. If the sum of the volume fractions of ferrite, graphite, and cementite is less than 95%, that is, the volume fraction of phases other than these exceeds 5%, workability is lowered. Moreover, when graphite rate is less than 5%, workability will fall.

Here, the volume ratio of ferrite, graphite, and cementite was calculated | required as follows. That is, after grinding the plate thickness 1/4 position of the plate thickness cross section in the rolling direction of the steel sheet, the nitrile is corroded, and 5 points per view and 10 views (total 50 points in total) at 400 times magnification by an optical microscope. ), And these images are subjected to image analysis processing using the image analysis software "Image Pro Plus ver.4.0" manufactured by Media Cybernetics, to determine the area of ferrite, graphite, and cementite, and to occupy the entire observation area (area ratio). ) Is the volume fraction of ferrite, graphite, and cementite. Moreover, the ratio (area rate) of the area Sgr of graphite to the sum of the area Sgr of graphite and the area Scm of cementite was made into the volume ratio (graphite rate) of graphite. That is, the graphite rate (%) can be represented by the following formula.

Graphite Rate = {Sgr / (Sgr + Scm)} × 100

Only by controlling the sum of the volume ratios of the ferrite, graphite, and cementite and the graphite rate, excellent hardenability, in particular, hardenability at the time of high frequency hardening is not obtained. That is, in this invention, in order to ensure the outstanding hardenability, it is necessary to make the average particle diameter of cementite and graphite into 5 micrometers or less. More preferably, it is 3 micrometers or less.

MEANS TO SOLVE THE PROBLEM The present inventors examined variously in order to acquire the outstanding hardenability. An example of examination is shown below. That is, C: 0.55%, Si: 0.01%, Mn: 0.10%, P: 0.003%, S: 0.0006%, Al: 0.005%, N: 0.0018%, Ni: 0.50%, B: 0.0013%, balance Fe and After heating the steel slab made of unavoidable impurities to 1150 ° C., five passes of rough rolling were performed, and seven passes of finish rolling was performed at a finishing temperature of 880 ° C. to obtain a hot rolled sheet having a thickness of 4.0 mm. After winding up at the temperature of 430 degreeC, it pickled and carried out the batch-anneal of 40-hr at 720 degreeC. At this time, in order to change the particle diameter of cementite and graphite, the temperature range from the finish rolling to the coiling temperature was changed in the range of air cooling (5 ° C / s) to 200 ° C / s and cooled. And the structure and hardenability were investigated as follows.

In the same manner as described above, after polishing and nitriding the plate thickness 1/4 position in the parallel cross section in the rolling direction, the scanning electron microscope was carried out over 10 fields of view (50 views in total) at a magnification of 1500 times at 5 points in each section and at each point. The center of the two points on the outer periphery of cementite or graphite and the corresponding ellipse (the same area as cementite and graphite, and whose primary and secondary moments are equal), using the image analysis software described above. The diameters passing through the weights were measured and averaged at 2 degree intervals, and the respective particle diameters were obtained. The particle diameters of cementite and graphite obtained by observing 50 visual fields were averaged to obtain an average particle diameter of cementite and graphite.

Hardenability: The disk test piece of diameter 100mm was extract | collected, and the outer periphery end of the disk test piece was heated to 1000 degreeC at the frequency of 100 Hz using the high frequency heat processing apparatus, and it cooled immediately. And at 8 positions along the circumferential direction of the disc test piece after heat treatment, Vickers hardness Hv [load: 49 N (= 5 kgf)] of the front and back surface 1.5 mm inside from the outer circumferential end was measured, and the maximum Hv and the minimum were measured. The difference ΔHv of Hv was obtained. When this (DELTA) Hv is 8 or less, it can be said that hardenability is excellent.

1 shows the relationship between the average particle diameter d of Cementite and graphite and ΔHv. When the average particle diameter d of cementite and graphite becomes 5 micrometers or less, it turns out that (DELTA) Hv becomes 8 or less and excellent hardenability is obtained.

As a result of various studies based on the above studies, the inventors have to make the average particle diameter of cementite and graphite be 5 µm or less, and more preferably 3 µm or less, in order to secure excellent hardenability. I found out that there is. The reason why excellent hardenability is obtained by defining a structure in this way is considered as follows. That is, it is considered that when cementite and graphite average particle diameters are 5 micrometers or less, cementite and graphite will melt | dissolve almost completely at the time of high frequency heating, and the uniformity of the hardness after hardening can be aimed at.

3) manufacturing condition

Below, the preferable manufacturing conditions of the steel plate of this invention are shown. In addition, the manufacturing method of the steel plate of this invention is not limited to the following.

Finishing temperature during hot rolling: 800 to 950 ℃

When the finishing temperature at the time of hot rolling is less than 800 degreeC, the increase of a rolling load becomes remarkable, and when it exceeds 950 degreeC, the produced scale will become thick and pickling property will fall, and a decarburization layer may produce | generate on a steel plate surface layer. Therefore, it is set to 800-950 degreeC.

Average cooling rate after hot rolling: 50 ° C / s or more

The steel sheet after hot rolling is cooled by the average cooling rate of 50 degrees C / s or more to the cooling stop temperature mentioned later immediately. If the average cooling rate is less than 50 ° C / s, growth of ferrite particles is likely to occur during cooling, and large ferrite particles are formed. In the subsequent annealing, graphite and cementite are thought to be formed by the ferrite grain boundary or inclusions as nuclei. Therefore, when the ferrite particles are large, the graphite and cementite formed as the nucleus grains become coarse and the hardenability decreases. . When the average cooling rate is low, coarse pearlite is formed, and graphite and cementite are formed through the division, coagulation, and coarsening of the pearlite. Therefore, graphite and cementite are coarsened and hardenability is reduced. When the average cooling rate is 50 ° C / s or more, the rolling strain introduced into the austenite by hot rolling tends to remain in the structure after transformation, resulting in an increase in dislocation density, and at the time of annealing, such dislocations into the nucleus. It also has the advantage of promoting the formation of graphite. From the above, the average cooling rate is at least 50 ° C / s, preferably at least 80 ° C / s. Although the upper limit of an average cooling rate does not need to be specifically defined, In order to suppress deterioration of a steel plate shape and ensure a steel plate shape, it is preferable to set it as 200 degrees C / s or less.

Cooling stop temperature in cooling after hot rolling: 500 degrees C or less

When the minimum temperature that needs to be cooled by the above cooling rate, that is, the cooling stop temperature exceeds 500 ° C, cornerstone ferrite is generated during cooling until winding, coarse pearlite is formed, and at the time of annealing after winding Since cementite and graphite become coarse and hardenability falls, it is 500 degrees C or less, Preferably it is 470 degrees C or less. Although the minimum of cooling stop temperature does not need to be specifically defined, In order to ensure the shape of a steel plate, it is preferable to set it as 200 degreeC or more.

Winding temperature: 450 ℃ or less

After cooling, the hot rolled sheet is immediately wound up. At that time, when the coiling temperature exceeds 450 ° C, coarse pearlite is formed, cementite and graphite are coarsened during annealing, and hardenability is reduced. Therefore, winding temperature shall be 450 degrees C or less. Moreover, in order to fully acquire the cooling effect after said hot rolling, it is preferable to make winding temperature into lower temperature than cooling stop temperature. Moreover, since the shape of a hot rolled sheet is easy to deteriorate, it is preferable to make winding temperature into 200 degreeC or more.

Annealing Temperature: Below 720 ℃

The hot rolled sheet after winding is subjected to annealing in order to promote spheroidization and graphite of cementite and to soften after descaling by pickling or the like. In that case, when annealing temperature exceeds 720 degreeC, coarse pearlite will generate | occur | produce during cooling and a hardenability will be reduced, and it shall be 720 degrees C or less. Moreover, when annealing temperature is less than 600 degreeC, since annealing time becomes extremely long, it is preferable to make annealing temperature 600 degreeC or more.

In addition, the annealing time does not need to be particularly limited, but in order to form graphite, it is 8 hr or more, and since ferrite particles are excessively coarsened and may cause ductility reduction, it is preferably 100 hr or less. desirable.

In order to solvent the steel of this invention, any of a converter and an electric furnace can be used. The steel melted in this way becomes a slab by ingot-division rolling or continuous casting. The slab is usually hot (reheated) and then hot rolled. In addition, in the case of the slab manufactured by continuous casting, you may apply the direct rolling rolling which keeps rolling, as it is, or for the purpose of suppressing temperature fall. When reheating a slab and hot rolling, it is preferable to make slab heating temperature 1280 degrees C or less in order to avoid deterioration of the surface state by a scale. Hot rolling can also be performed only by finish rolling, omitting rough rolling. In order to ensure the finishing temperature, the rolled material may be heated by heating means such as a sheet bar heater during hot rolling. The thickness of the hot rolled sheet is not particularly limited as long as the production conditions of the present invention can be maintained, but is preferably 1.0 to 10.0 mm. The steel sheet after annealing can be temper rolled as needed. Example is shown in Example 1.

Only by controlling the sum of the volume fractions of the ferrite, graphite, and cementite, and the graphite rate, excellent elongation flange properties are not necessarily obtained. That is, in this invention, in order to ensure the outstanding elongation flange property, it is necessary to make the total volume ratio of cementite and graphite which exist in a ferrite particle into 15% or less. More preferably, it is 10% or less.

MEANS TO SOLVE THE PROBLEM The present inventors performed various examination in order to acquire the outstanding elongation flange property. An example of examination is shown below. C: 0.55%, Si: 0.01%, Mn: 0.10%, P: 0.003%, S: 0.0006%, Al: 0.005%, N: 0.0018%, Ni: 0.50%, B: 0.0013%, balance Fe and inevitable After heating the steel slab made of impurity to 1150 ° C, 5 passes of rough rolling are performed, and 7 passes of finish rolling is performed at a finishing temperature of 870 ° C to form a hot rolled sheet having a sheet thickness of 4.0 mm, and wound at a winding temperature of 520 ° C. After that, it was pickled and subjected to batch anneal of 40 hr at 720 ° C. At this time, in order to change the quantity and distribution state of cementite and graphite, the temperature range from the finish rolling to the coiling temperature after cooling was changed and cooled in the range of air cooling (5 degree-C / s)-200 degreeC / s. Then, the structure and stretch flangeability were examined as follows.

In addition, as described above, the plate thickness 1/4 position of the parallel cross section in the rolling direction was observed with an optical microscope over 10 visual fields (total 50 visual fields) at a magnification of 400 times at 5 points and 5 points in each section after polishing and nitriding corrosion. By using the above-described image analysis software, the cementite and graphite present in the ferrite grain boundary and the cementite and graphite present in the ferrite grain are identified, and the occupied area S _on the cementite and graphite present in the ferrite grain boundary and ferrite particles The occupied area S _in of cementite and graphite present in the particles is measured, and the area ratios of cementite and graphite present in the ferrite particles are obtained from the following equation, and the volume fractions of cementite and graphite present in the ferrite particles occupying the whole cementite and graphite It was set as S (%). That is, S (%) can be represented by the following formula.

S = {S _in / (S _on + S _in )} × 100

Here, the cementite particles or graphite particles having a portion present at least partially on the ferrite grain boundary have the area of the entire cementite or graphite particle as the occupied area of the cementite particles or graphite particles present on the ferrite grain boundary. It measured and measured the area of the cementite or graphite particle which does not have the part which exists on a ferrite grain boundary as the occupation area of the cementite particle or graphite particle which exists in a ferrite particle.

Elongation flangeability: The test piece for hole expansion test (100 * 100 mm) was extract | collected, and it punched using the punching tool of 10 mm of punch diameters, and 11.6 mm of die diameters (clearance: 20% of plate thickness) to the center of a test piece. Thereafter, the punched hole is pushed up with a cylindrical flat punch (diameter 50 mm Φ, shoulder R 8 mm) to expand the hole, and the hole diameter d (mm) at the time point at which the plate thickness through crack occurs at the hole edge is measured. Then, the hole expansion ratio λ (%) was calculated from the following equation, and the same test was performed six times to obtain an average λ (%).

λ = 100 × (d-10) / 10

2 shows the relationship between the volume fraction S of the cementite and graphite present in the ferrite particles and the average lambda. When the volume fraction S of cementite and graphite existing in the ferrite particles is 15% or less, an average lambda of 60% or more is obtained, and it is understood that excellent elongation flangeability is obtained.

As a result of various studies based on the above studies, the inventors have to make the total volume fraction of cementite and graphite present in the ferrite particles to be 15% or less in order to ensure excellent elongation flangeability, more preferably. Finds that it needs to be less than 10%. The reason for obtaining a good elongation flange property by defining a structure | tissue in this way is considered as follows. That is, when a large amount of cementite or graphite is present in the ferrite particles, fine cracks are likely to occur at the interface between cementite and graphite and ferrite during punching, propagating and bonding from the beginning during the hole expansion test to penetrate the plate thickness. Easy to lead to cracks. On the other hand, since the diffusion rate of carbon is faster at the ferrite grain boundaries, cohesion coarsening is promoted more than in the ferrite grains, and cementite and graphite on the ferrite grains tend to be coarser than those in the ferrite grains, and thus the intervals between the cementite grains and the graphite grains. It is easy to widen. For this reason, cementite and graphite on the ferrite grain boundary slow the crack propagation as compared with cementite and graphite in the ferrite grains.

3) manufacturing condition

Finishing temperature during hot rolling: 800 to 950 ℃

Average cooling rate after hot rolling: 50 ° C / s or more

When the steel sheet after hot rolling is cooled to the cooling stop temperature mentioned later immediately at an average cooling rate of 50 degrees C / s or more, formation of a cornerstone ferrite is suppressed and ferrite and cementite are minutely precipitated. As a result, C easily diffuses into the ferrite grain boundary during annealing performed after winding, and the coagulation and coarsening of cementite on the ferrite grain boundary is promoted, thereby increasing the cementation in the ferrite grains, thereby reducing the elongation flangeability. This is improved. In addition, the rolling deformation introduced into the austenite by hot rolling tends to remain in the structure after transformation, resulting in an increase in dislocation density. As a result, graphite formation using dislocation as the nucleus at the time of annealing becomes easy, softening progresses, and workability improves. From the above, the average cooling rate is at least 50 ° C / s, preferably at least 80 ° C / s. Although the upper limit of an average cooling rate does not need to be specifically defined, In order to suppress deterioration of a steel plate shape and ensure a steel plate shape, it is preferable to set it as 200 degrees C / s or less.

Cooling stop temperature in cooling after hot rolling: 600 degrees C or less

When the minimum temperature that needs to be cooled by the above cooling rate, that is, the cooling stop temperature exceeds 600 ° C, cornerstone ferrite is generated during cooling up to winding up, pearlite is generated, and ferrite particles are formed during annealing after winding up. Since cementite and graphite which exist in an inside increase and the extension | stretching flange property falls, it is 600 degrees C or less, Preferably it is 550 degrees C or less. Although the minimum of cooling stop temperature does not need to be specifically defined, In order to ensure a steel plate shape, it is preferable to set it as 200 degreeC or more.

Winding temperature: below 550 ℃

After cooling, the hot rolled sheet is immediately wound up. At that time, when the coiling temperature exceeds 550 ° C., pearlite is formed, and cementite and graphite present in the ferrite grains at the time of annealing increase to decrease the elongation flangeability. Therefore, winding temperature shall be 550 degreeC or less. Moreover, in order to fully acquire the cooling effect after said hot rolling, it is preferable to make winding temperature into lower temperature than cooling stop temperature. Moreover, since the shape of a hot rolled sheet tends to deteriorate, in order to ensure the shape of a steel plate, it is preferable to make winding temperature into 200 degreeC or more, More preferably, it is more than 450 degreeC.

Annealing Temperature: Below 720 ℃

The hot rolled sheet after winding is subjected to annealing in order to promote spheroidization and graphite formation of cementite and to soften after descaling by pickling or the like. In that case, when annealing temperature exceeds 720 degreeC, a pearlite will generate | occur | produce during cooling and it will make extension flange property fall, and shall be 720 degrees C or less. Moreover, when annealing temperature is less than 600 degreeC, since cementite and graphite which exist in a ferrite particle tend to increase, and an extension flange property deteriorates, it is preferable to make annealing temperature 600 degreeC or more.

In addition, the annealing time does not need to be particularly limited, but in order to form graphite and to reduce cementite and graphite in the ferrite particles, the annealing time is 8 hr or more, and there is a fear that the ferrite particles are excessively coarsened to cause ductility reduction. Since it exists, it is preferable to set it as 100 hr or less.

In order to melt the steel of this invention, any of a converter and an electric furnace can be used. The steel melted in this way becomes a slab by ingot-division rolling or continuous casting. The slab is usually hot (reheated) and then hot rolled. In addition, in the case of the slab manufactured by continuous casting, you may apply the direct rolling rolling which keeps rolling, as it is, or for the purpose of suppressing temperature fall. When reheating a slab and hot rolling, it is preferable to make slab heating temperature 1280 degrees C or less in order to avoid deterioration of the surface state by a scale. Hot rolling can also be performed only by finish rolling, omitting rough rolling. In order to ensure the finishing temperature, the rolled material may be heated by heating means such as a sheet bar heater during hot rolling. The thickness of the hot rolled sheet is not particularly limited as long as the production conditions of the present invention can be maintained, but is preferably 1.0 to 10.0 mm. The hot rolled sheet is annealed after removing the scale of the surface by pickling or shot blasting. The steel sheet after annealing can be temper rolled as needed. Example is shown in Example 2.

Example

Example 1

The slab of steel No.A-S of the composition shown in Table 1 is heated at 1250 degreeC, hot-rolled on the conditions shown in Table 2, and after pickling, it is annealed on the conditions shown in Table 2 similarly, and plate thickness is 4.0 mm. Phosphorous steel sheets Nos. 1 to 22 were produced. And by the said method, (DELTA) Hv which evaluated the graphite rate, the average particle diameter of cementite and graphite, and hardenability was calculated | required. Moreover, the JIS No. 5 tensile test piece was extract | collected along the rolling direction, the tensile test was done, and yield stress YP, tensile strength TS, and elongation El were calculated | required.

The results are shown in Table 3. It is understood that the steel sheets of the present invention are all low YP, low TS, high El, low ΔHv, soft, excellent in workability and excellent in hardenability. In addition, as shown in Table 3, the structure of the steel plate of the example of this invention consisted mostly of ferrite, cementite, and graphite, and it confirmed that these total volume ratios were 95% or more.

Example 2

The slab of steel No.AA-AS of the composition shown in Table 4 is heated at 1250 degreeC, hot-rolled on the conditions shown in Table 5, and after pickling, it is annealed on the conditions shown in Table 5 similarly, and plate thickness is 4.0 mm. Phosphorus steel plates No. 101 to 122 were produced. Then, by the above method, the graphite ratio, the volume fraction S of cementite and graphite present in the ferrite particles occupied in the whole cementite and graphite, and the average? Of the stretch flangeability were determined. Moreover, JIS No. 5 tensile test piece was extract | collected along the rolling direction, the tensile test was done, and yield stress YP, tensile strength TS, and elongation El were calculated | required. In addition, the same test was done twice with respect to each sample, and the average value was calculated | required, and this average value was made into the characteristic value of the steel plate.

The results are shown in Table 6. It is understood that the steel sheets of the present invention are all low YP, low TS, high El, high λ, soft, and have excellent workability including stretch flangeability. In addition, as shown in Table 6, the structure of the steel plate of the example of this invention consisted mostly of ferrite, cementite, and graphite, and it confirmed that these total volume ratios were 95% or more.

Claims

In mass%, C: 0.3-0.7%, Si: 0.1% or less, Mn: 0.20% or less, P: 0.01% or less, S: 0.01% or less, Al: 0.05% or less, N: 0.0050% or less, and It has a composition consisting of balance Fe and unavoidable impurities, has a structure containing ferrite, graphite, and cementite, and the total volume fraction of ferrite, graphite, and cementite in the whole structure is 95% or more, and graphite in the whole graphite and cementite A steel sheet having a volume fraction (graphite ratio) of 5% or more and an average particle diameter of graphite and cementite of 5 µm or less.

The method of claim 1,
Furthermore, the steel sheet which has a composition containing at least 1 sort (s) chosen from mass: Ni: 3.0% or less, B: 0.005% or less, Cu: 0.1% or less.

The steel having the composition according to claim 1 or 2 is hot rolled at a finishing temperature of 800 to 950 ° C. to form a hot rolled sheet, and the hot rolled sheet after the hot rolling is 500 ° C. or lower at an average cooling rate of 50 ° C./s or more. It winds up to the winding temperature of 450 degrees C or less after cooling to the cooling stop temperature of the, and the manufacturing method of the steel plate characterized by annealing at the annealing temperature of 720 degrees C or less after the said hot rolled sheet.

In mass%, C: 0.3-0.7%, Si: 0.1% or less, Mn: less than 0.15%, P: 0.01% or less, S: 0.01% or less, Al: 0.05% or less, N: 0.0050% or less, and It has a composition consisting of balance Fe and unavoidable impurities, has a structure containing ferrite, graphite, and cementite, and the total volume fraction of ferrite, graphite, and cementite in the whole structure is 95% or more, and graphite in the whole graphite and cementite The steel sheet having a volume fraction (graphite ratio) of 5% or more and a sum of the volume fractions of graphite and cementite present in the ferrite grains occupying the graphite and cementite as a whole is 15% or less.

The method of claim 4, wherein
Furthermore, the steel sheet which has a composition containing at least 1 sort (s) chosen from mass: Ni: 3.0% or less, B: 0.005% or less, Cu: 0.1% or less.

The steel having the composition according to claim 4 or 5 is hot rolled at a finishing temperature of 800 to 950 ° C to form a hot rolled plate, and the hot rolled plate after the hot rolling is 600 ° C or less at an average cooling rate of 50 ° C / s or more. It winds up to the winding temperature of 550 degreeC or less after cooling to the cooling stop temperature of the, and the manufacturing method of the steel plate characterized by annealing at the annealing temperature of 720 degreeC or less after the said hot rolled sheet.